Lusters are the elves' currency. At birth, every elf is given a birth fund which has 5 million lusters, with 1 luster equal to about 1 million human dollars, making each birth fund worth about $5,000,000,000,000 in human money. Lusters are stored in a green cube carried by each elf. To pay lusters to another elf, one would swipe the green cube against a machine.
Although most elves obtain their funds at birth, Sophie received hers when she first arrived at the Lost Cities in Book 1: Keeper of the Lost Cities since she was born in the Forbidden Cities to a human family.